The present invention relates generally to a method for reducing the entrapment of lubricating oil in the tubing of an air conditioner heat exchanger by coating the tubing with a low surface energy coating, improving the heat transfer in evaporators and condensers.
During operation of an air conditioner or other refrigerant cycle, lubricating oil in the compressor may leak and mix with the refrigerant that circulates through the air conditioning system. As the refrigerant flows through the tubing of the evaporating and condensing heat exchangers, the lubricating oil coats and wets the inner surface of the heat exchangers.
Often, the inner surface of the tubing of a heat exchanger is provided with interstices to increase the effective area for heat transfer. As the refrigerant flows through the evaporator, the lubricating oil mixed with the refrigerant is easily entrapped in the interstices of the tubing, smoothing the inner surface and reducing the effective area for heat transfer. Additionally, the tubing of the evaporating heat exchanger is commonly made of copper or aluminum which easily oxidizes to form an oxide layer having a high surface energy. As the oxide layer has a high surface energy, the oxide layer wets well, further causing the lubricating oil to adhere as a film on the inner surface of the evaporator. Additionally, if the condenser is enhanced, the layer of lubricating oil is further encouraged by entrapment in the textured surface formed by the interstices. To improve heat transfer, it is preferred that the lubricating oil form droplets rather than a film on the inner surface of the heat exchangers of an air conditioner.
Hence, there is a need in the art for a method for reducing oil entrapment on the inner surface of the tubing of a heat exchanger of an air conditioner.
The present invention relates to a method for reducing oil entrapment on the inner surface of the tubing of a heat exchanger of an air conditioner.
A thin coating of a lower surface energy material in solution is applied on the inner surface a condenser or an evaporator. The solution is applied by running the solution through the tubing of the heat exchanger. After the solution is drained from the tubing and the inner surface of the heat exchanger is dried, a monomolecular layer of the material in solution remains on the inner surface of the heat exchanger.
Any polymer with a lower surface energy and having a chemical and thermal resistance can be employed in the solution. Preferably, silane, fluorocarbons, polyetheretherketon (PEEK) and polysulfones are utilized in the solution as these polymers have lower surface energies and will coat and adhere to the inner surface of the tubing. Most preferably, silane is employed in low concentrations of 1-2% by weight as the polymer in the lower surface energy solution.
By applying a thin coating of a lower surface energy material to the inner surface of either an evaporating or condensing heat exchanger, lubricating oil in the compressor which mixes with the refrigerant circulating through the air conditioning system will not wet and form a film over the higher surface energy oxide coated inner surface of the heat exchanger. As the thin coating of the lower surface energy material on the inner surface of the heat exchanger has a lower surface energy, droplets of lubricating oil will form. By preventing the wetting of lubricating oil on the inner surface of the evaporator, heat transfer is improved.
Accordingly, the present invention provides a method for reducing oil entrapment on the inner surface of the tubing of a heat exchanger of an air conditioner.
These and other features of the present invention will be best understood from the following specification and drawings.